Chlorinated alkenes are important industrial chemicals that are useful in manufacture of a wide variety of chlorinated polymers and organic chemicals. For example, chlorinated alkenes may be utilized as monomers in polymerization reactions that yield thermoplastic and elastomeric compositions. Such materials are useful in manufacture of adhesives and molded and extruded goods, such as gaskets, hoses and thermoplastic profiles.
In many instances production of chlorinated alkenes, including chlorinated alkene monomers, involves dehydrochlorination of chlorinated alkanes or other chlorinated alkenes as one step of a reaction sequence. A typical dehydrohalogenation process performed in the presence of a phase transfer catalyst is described in U.S. Pat. No. 3,981,937 to Campbell et al., wherein 3,4-dichlorobutene-1 is dehydrochlorinated with aqueous alkali to form 2-chlorobutadiene-1,3 (i.e. chloroprene). Other examples of dehydrochlorination reactions are disclosed in, for example, U.S. Pat. No. 4,629,816, which discloses a process for dehydrochlorination of 2,3,4-trichlorobutene-1 to form 2,3-dichloro-1,3-butadiene and in U.S. Pat. No. 2,626,964, which discloses a method for dehydrohalogenation of 1,2,3,4-tetrachlorobutane to form 2,3-dichloro-1,3-butadiene.
Although phase transfer catalysts are very effective at increasing conversion in some dehydrochlorination reactions, these higher conversions can result in increased formation of byproduct isomers that are difficult or impossible to remove from the desired product. In the dehydrochlorination of 3,4-dichlorobutene-1 to produce chloroprene for example, the reaction of some impurities in the organic reactant forms byproduct chlorobutadienes that contain chlorine substituents located on an alpha carbon atom (i.e. “alpha-chlorine”). As used herein, the term “alpha carbon atom” means a carbon at the end of a carbon chain (either an alkyl or alkenyl chain) generally numbered 1 in the IUPAC naming convention for alkanes and alkenes. By extension, a beta carbon atom is a carbon atom at the penultimate end of a carbon chain (“next to last” or second) generally numbered 2 in the IUPAC naming convention for alkanes and alkenes.
One method of controlling the problem of byproduct formation is by intentionally limiting reactant conversion to a level below that which is otherwise achievable. However, although intentionally limiting reactant conversion allows acceptable product purity to be attained, it also reduces yield and imposes an economic and environmental disadvantage that can be significant. In another example, high conversion conditions in the dehydrochlorination of 1,2,3,4-tetrachlorobutane to form 2,3-dichlorobutadiene-1,3 results in production of substantial amounts of isomeric dichlorobutadienes that contain alpha-chlorine. The presence of high levels of isomeric products that contain alpha-chlorine is objectionable because use of such mixtures as monomer feeds in polymerization reactions can result in formation of relatively high percentages of allylic chlorine in the polymer backbone. This can increase oxidative degradation of the polymer.
When a polymerization is terminated at a low conversion level, it is necessary to remove and desirable to recycle unreacted monomers prior to isolation of the polymeric product. One undesirable aspect of the monomer recovery is that certain impurities that may be present in the monomeric starting material may be concentrated in the recovered monomer stream as a result of chemical reactions that take place during polymerization or conditions that exist in the reactor. If the recovered monomer is recycled to the reactor without treatment, abnormal polymerization initiation and polymers having inferior properties will typically result. Therefore, it is desirable to reduce the level of impurities in the recovered monomer, generally in a separate step during the recycle operation in order to prevent this problem. This is especially true for recycled chloroprene monomer which contains higher than desired levels of 1-chlorobutadiene-1,3.
Removal of isomers having alpha-chlorine substituents from a desired isomeric product that contains chlorine substituents exclusively on beta carbon atoms is very difficult because the isomers generally have similar volatility. In recycle operations, excessive losses of the desired isomer can occur when typical distillation techniques are employed to separate the undesired isomers from recovered reactant streams. For this reason, conversion in some dehydrochlorination reactions may be intentionally limited to levels lower than those that are achievable in order to reduce formation of isomers containing alpha-chlorine substituents to acceptable levels to avoid the necessity of expensive purification steps.
It would be advantageous to have a simple method available that would permit the purification and separation of desired chlorinated olefins that contain beta-chlorine substitutents from mixtures of such compounds with other chlorinated isomers. This would permit the use of high conversion processes in dehydrochlorination reactions without compromising final product purity and provide for the efficient use of recovered monomers for the production of polymers.